Folding knives are extremely popular among sportsmen, members of the police force, firemen or other emergency service personnel whose duties may sometimes require the one-handed use of a cutting tool. The folding knife is particularly advantageous for such applications because it occupies a small amount of space in one's pocket and also it is totally safe to handle when in folded condition as the blade is securely stored in the handle.
A typical folding knife comprises a hollow handle and a blade retained to the handle by a pivot pin allowing the blade to move between a closed and an opened position. In the closed position, the cutting edge of the blade is securely received within the cavity in the handle, whereas in the opened position the blade is in axial alignment with the handle, the cutting edge of the blade being exposed. A spring loaded latch mechanism is provided in the handle to lock the blade in the opened position to prevent an accidental retraction thereof while the knife is being used. An actuator is provided on the handle to disengage the spring latch when it is desired to fold the knife.
In order to open the blade, it is known to provide a groove or notch near the non-cutting edge of the blade to be grasped by fingers to pivot the blade out of the handle. This arrangement requires two hands to open the blade which may constitute a disadvantage in a situation where one is holding an article to be cut with one hand, thereby leaving free only the other hand to open the knife.
To address this problem, various devices have been developed and commercialized in past years. One example is the so called switch blade knife which has a spring loaded blade that can be extended by depressing an actuator on the handle. Although this arrangement allows a one-hand operation, this type of knife is considered as an illegal weapon in most places.
Another example is the provision of a thumb actuator in the form of a pin secured near the non-cutting edge of the blade close to the pivot pin between the blade and the handle. Normally, the pin is mounted into a hole drilled in the blade. Although this system allows a one-hand operation, it is extremely difficult to install on an existing knife as the blade is made of a hardened material and is very difficult to drill. For this practical reason, thumb actuators of this type are installed only at the manufacturing stage of the knife, before the blade has been subjected to any hardening treatment. This also draws another disadvantage in that the manufacturer selects the location of the pin on the blade which may not necessarily suit the user preference.
A further example is the spring clip thumb actuator of the type described in the Pharr U.S. Pat. No. 4,095,337 which has been issued on June 20, 1978. This actuator comprises a resilient steel saddle tightly fitted over the non-cutting edge of the blade and a flat plate mounted to the saddle to be engaged by the thumb to open the blade. This arrangement has two disadvantages. Firstly, it cannot be used equally well by left and right handed individuals. Secondly, it lacks adjustability as it attaches well, only where the blade thickness exceeds by a slight amount the spacing of the resilient steel saddle.